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When law enforcement officials searched through Stephen Paddock’s arsenal of weapons, they discovered that 12 of his 47 firearms were outfitted with a deadly device that allows a semiautomatic weapon to perform more like a fully automatic weapon.

The device used to modify the firearm is known as a “bump stock.” Officials say it allows firearms to fire hundreds of rounds per minute.

A bump stock allows the body of the rifle to slide a short distance back and forth, harnessing the recoil energy of each shot. The shooter does not move the trigger finger; instead, the weapon bounces, or “bumps,” rapidly between shoulder and finger.

Sen. Dianne Feinstein (D-Calif.), who has been working on legislation that would ban bump stock modifications, said on Twitter that the device allows semiautomatic weapons to fire “up to 800 rounds per minute ― the rate of automatic weapons ― and inflict maximum carnage.”

In the wake of the deadliest mass shooting in U.S. history, Congress must examine ways to prevent tragedies like this from happening in the future.

Automatic weapons are illegal, but individuals are still able to purchase bump stocks to allow semi-automatic weapons to fire up to 800 rounds per minute—the rate of automatic weapons—and inflict maximum carnage.

At least 58 people were killed and more than 500 were injured as bullets rained down on the concertgoers. The rapid firing of the bullets caused many witnesses to believe the gunman was using a fully automatic weapon.

“All of a sudden we heard what sounded like a machine gun and people just started screaming that they were hit and to get down,” witness Megan Kearny told NBC News. “About every 20 seconds after that you would hear a round of machine guns, and people just dropping, hundreds of bodies over the ground.”

Jill Snyder, special agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives, told reporters on Tuesday that 12 weapons found in Paddock’s hotel room had “bump fire stock” devices, though she did not say which firearms were used in the shooting.

“Bump fire stock, while simulating automatic fire, do not actually alter the firearm to fire automatically, making them legal under current federal law,” Snyder said.

Bump stocks are legal, according to the ATF, because they do not permanently modify the firearm nor do they constitute a mechanical modification of the device. However, they remain highly controversial.

In an interview with The Associated Press, Erich Pratt, executive director of Gun Owners of America, defended the use of bump stock devices, which gun advocates say they use for entertainment.

“Ultimately, when Congress... looks at this, they’ll start asking questions about why anybody needs this, and I think the answer is we have a Bill of Rights and not a Bill of Needs,” Pratt told the AP.

“To those who say we can’t talk about machine gun massacres right after the massacre: I’m done waiting for the ‘right time’ to talk about it,” Inslee said in a statement Tuesday. “The ‘can’t talk about it now’ crowd is killing us.”